1. Field of the Invention
The present invention relates to an aluminum electrolytic capacitor and a method for producing the same.
2. Description of the Related Art
When an aluminum electrolytic capacitor is left in a high-temperature atmosphere without a load, the capacity thereof is decreased and gas is generated to increase an internal pressure, which leads to leakage of liquid. This phenomenon is caused by corrosion of a cathode foil made of aluminum, and becomes more conspicuous in a system with water added to an electrolyte. Furthermore, in the case where an aluminum electrolytic capacitor is left at a high temperature under the application of a voltage, an anode lead made of aluminum is subjected to electrolytic etching, which eventually results in disconnection.
As a method for preventing such corrosion of aluminum, a method for treating the surface of aluminum with phosphoric acid to form a coating of aluminum phosphate is known conventionally (see JP 62 (1987)-17185 A). According to this method, by previously forming a coating of aluminum phosphate, a corrosion reaction (Alxe2x86x92Al3++3exe2x88x92) is prevented.
However, even if such treatment is conducted, corrosion may not be prevented sufficiently. Particularly, in the case where a moisture content of an electrolyte is increased, corrosion is likely to occur.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an aluminum electrolytic capacitor with high reliability and a method for producing the same.
In order to achieve the above-mentioned object, the aluminum electrolytic capacitor of the present invention includes: a case; a sealant for sealing the case; a separator, a cathode, an anode, and an electrolyte sealed in the case; and two leads respectively connected to the cathode and the anode, wherein the cathode and the anode include an aluminum foil, the leads are made of aluminum, and a solid compound for keeping a pH of the electrolyte constant further is provided in the case. According to this configuration, the pH of the electrolyte can be prevented from being decreased too much, so that an aluminum electrolytic capacitor with high reliability can be obtained.
In the above-mentioned aluminum electrolytic capacitor, pits may be formed on a surface of at least one electrode selected from the cathode and the anode, and the solid compound may be disposed in the pits. According to this configuration, in particular, the corrosion of an electrode can be prevented.
In the above-mentioned aluminum electrolytic capacitor, the solid compound may adhere to the separator. According to this configuration, in particular, corrosion of an electrode can be prevented.
In the above-mentioned aluminum electrolytic capacitor, the solid compound may adhere to a surface of the lead. According to this configuration, in particular, corrosion of a lead can be prevented.
In the above-mentioned aluminum electrolytic capacitor, the solid compound may contain metal, and an oxidation potential EM at which the metal changes to positive ions and an oxidation potential EA at which aluminum changes to aluminum ions may satisfy a relationship: EMxe2x89xa6EA.
In the above-mentioned aluminum electrolytic capacitor, the solid compound may include at least one compound selected from the group consisting of an oxide of yttrium, a hydroxide of yttrium, an oxide of aluminum, a hydroxide of aluminum, an oxide of scandium, a hydroxide of scandium, an oxide of lanthanum, a hydroxide of lanthanum, an oxide of praseodymium, a hydroxide of praseodymium, an oxide of neodymium, a hydroxide of neodymium, an oxide of promethium, a hydroxide of promethium, an oxide of cerium, a hydroxide of cerium, an oxide of gadolinium, a hydroxide of gadolinium, an oxide of terbium, a hydroxide of terbium, an oxide of dysprosium, a hydroxide of dysprosium, an oxide of holmium, a hydroxide of holmium, an oxide of erbium, a hydroxide of erbium, an oxide of thulium, a hydroxide of thulium, an oxide of lutetium, a hydroxide of lutetium, an oxide of beryllium, and a hydroxide of beryllium. According to this configuration, an aluminum electrolytic capacitor with particularly high reliability can be obtained.
Furthermore, the first production method of the present invention is a method for producing an aluminum electrolytic capacitor including a cathode and an anode, including the processes of: (i) soaking an aluminum foil with pits formed on a surface thereof in an aqueous solution of a nitrate of metal; and (ii) after the process (i), soaking the aluminum foil in an alkaline aqueous solution to form a hydroxide of the metal in the pits, thereby forming at least one electrode selected from the cathode and the anode, wherein an oxidation potential EM at which the metal changes to positive ions and an oxidation potential EA at which aluminum changes to aluminum ions satisfy a relationship: EMxe2x89xa6EA. According to this production method, a highly reliable electrolytic capacitor can be produced in which an electrode is unlikely to corrode.
According to the above-mentioned first production method, the metal may include at least one metal selected from the group consisting of yttrium, aluminum, scandium, lanthanum, praseodymium, neodymium, promethium, cerium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, lutetium, and beryllium.
According to the above-mentioned first production method, a pH of the alkaline aqueous solution is in a range of 8 to 12.
Furthermore, a second production method of the present invention is a method for producing an aluminum electrolytic capacitor including a cathode and an anode, including the processes of: (I) soaking an aluminum foil with pits formed on a surface thereof in an aqueous solution of a nitrate of metal; and (II) after the process (I), heat-treating the aluminum foil to form an oxide of the metal in the pits, thereby forming at least one electrode selected from the cathode and the anode, wherein an oxidation potential EM at which the metal changes to positive ions and an oxidation potential EA at which aluminum changes to aluminum ions satisfy a relationship: EMxe2x89xa6EA. According to this production method, a highly reliable electrolytic capacitor can be produced in which an electrode is unlikely to corrode.
According to the above-mentioned second production method, the metal may include at least one metal selected from the group consisting of yttrium, aluminum, scandium, lanthanum, praseodymium, neodymium, promethium, cerium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, lutetium, and beryllium.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.